Structurally Engineered And Electrochemical Performance Research Of Carbon Anode Materials For Sodium/potassium Ion Batteries

The limited lithium resources and uneven geographical distribution have seriously hindered the further development and large-scale application of lithium ion batteries(LIBs).In contrast,sodium and potassium have the advantages of extensive resources and low cost,so sodium ion batteries(SIBs)and potassium ion batteries(PIBs)are considered to be new secondary batteries that are expected to replace lithium ion batteries in the field of large-scale energy storage.Carbon material with a stable structure,simple preparation process,and environmental friendliness,is the most promising anode materials.In this paper,energy storage carbon materials with suitable structures are prepared for different structural characteristics of SIBs/PIBs carbon anode materials.By adjusting the disorder degree and interlayer spacing of the microcrystalline structure,biomass-based hard carbon with high sodium storage capacity and high first coulombic efficiency was prepared;comparing the structural characteristics of graphite with different expansion coefficients and different layer spacings in the buffer potassium storage process,a potassium anode graphite anode material with high potassium storage capacity and excellent cycle stability was obtained;using polystyrene microspheres as templates and polydopamine as nitrogen-containing precursors,nitrogen-doped hollow carbon nanospheres were constructed.The advantage of the hollow spherical shell structure in the volume expansion of the buffer material is used to improve the cycle stability of the carbon material during potassium storage.(1)Preparation and performance study of biomass-based hard carbon anode materials for sodium ion batteries.Sawdust are used as precursors to prepare hard carbon materials by carbonized at high temperature,and the graphite microcrystal structure and layer spacing are adjusted by changing the carbonization temperature.As the carbonization temperature increasing,the structural regularity of the carbon material increases,the interlayer spacing decreases,and surface defects decrease.The PT-1200 layer spacing of the sample prepared at 1200? is 0.38 nm,and the specific surface is only 5.34 m2 g-1.It had a sodium storage capacity of 392.2 mAh g-1 at a current density of 30 mA g-1 and the first Coulombic efficiency can reach 82.2%.In addition,thanks to its large interlayer spacing and good structural stability,the PT-1200 electrode also exhibits excellent cycle performance,and the sodium storage capacity can maintained 224.8 mAh g-1 after 300 cycles at a current density of 50 mA g-1,which indicates that Sawdust-based hard carbon material is a good anode material for SIBs.(2)The electrochemical performance of expanded graphite anode materials for PIBs.Graphite is a commercial anode material for lithium-ion batteries.It has a mature preparation process and excellent electrochemical performance.However,since the radius of K+is larger than that of Li+(1.38 A vs.0.76 A),a large deformation pressure will be generated during the insertion-extraction process,resulting in a large volume expansion of graphite during potassium storage and a severe capacity decay.Expanded graphite with its large interlayer spacing and disordered porous structure can relieve the volume expansion during K+insertion-extraction process to improve the cycling stability of PIBs.Expanded graphites with different expansion coefficients were selected to compare the structure and electrochemical performance differences of different graphite materials,and to analyze the correlation between the structure and performance of graphite materials used as anodes for PIBs.The 300-fold expanded graphite(EG-300)has an interlayer spacing of 3.38 A and an IG/ID value of 3.22,which structure is relatively disordered.It has an initial potassium reversible capacity of 370.4 mAh g-1 with 61.9%efficiency in the first Cullen cycle,and 80.3%capacity retention after 250 cycles at 0.1 C,showing good cycle performance,indicating that by increasing the interlayer distance of the graphite material and increasing its degree of disorder,the graphite material can effectively improve the potassium storage structural stability during the process.(3)Preparation of nitrogen-doped hollow carbon nanospheres and electrochemical potassium storage properties.Using polystyrene microspheres as templates and polydopamine as nitrogen-containing carbon precursor,a novel nitrogen-doped hollow carbon nanosphere(NHCS-X)was constructed.The nitrogen heteroatoms and well-developed pore structure can provide abundant active sites for potassium storage.The unique hollow structure can effectively buffer the volume expansion of the potassium storage process,shorten the K+diffusion path.Therefore,NHCS-X is expected to show high capacity and excellent cycle and rate performance.By changing the carbonization temperature,the nitrogen doping content of the NHCS-X material,the layer spacing,the thickness of the spherical shell and the pore structure of the hollow spheres can be adjusted.The sample prepared at 800?(NHCS-800)has a nitrogen content of 3.82%and a specific surface of 225.4 m2 g-1.Using NHCS-800 as an anode material for PIBs at room temperature,the potassium storage specific capacity reached 396.2 mAh g-1 at a current density of 0.1 30 mA g-1.The hollow carbon nanosphere structure further enabled NHCS-800 to maintain high cycle stability with a capacity retention of 74.4%after 130 cycles at a current density of 50 mA g-1.In addition,when the current density is increased to 1000 mA g-1,the reversible capacity can still maintain 155.9 mAh g-1.This indicates that nitrogen-doped hollow carbon nanospheres are a carbon anode material with good electrochemical performance for PIBs.